Improve cache loading performance

[octo]

LogicMail currently loads all cached message headers from a folder before attempting to synchronize with the mail server. This process can be sufficiently slow to be annoying, especially with #211 likely leading to a dramatic increase in cached content.

The whole loading process needs to be profiled, bottlenecks found, and the user impact reduced.

Some ideas include:

• Triggering the server flag sync at the same time as the cache load, since cache file names do indicate message UIDs
• Speeding up the load in one of the following ways:
  • Finding/fixing deserialization bottlenecks
  • File data compression
  • Folder index files
• Maintaining a second cache in the persistent store
• Not loading all message headers until the user starts to scroll

[octo]

Implemented through a complete rewrite and optimization of the folder message cache, using the persistent store and Persistable container objects.